In hard disk drive applications, there is a constant drive to increase the recording areal density to reduce the cost of information storage. The increase in recording areal density is accomplished by decreasing the size of the writer and reader sensors that are used to record and reproduce signals. In today's products, the reader sensor is typically made using a TMR sensor structure, which includes two ferromagnetic layers that are separated by a dielectric layer called a tunnel barrier. One of the ferromagnetic layers is referred to as a reference layer (RL) wherein the magnetization direction is fixed by exchange coupling with an adjacent antiferromagnetic (AFM) pinning layer. The second ferromagnetic layer is a free layer (FL) wherein the magnetization vector can rotate in response to external magnetic fields to be either parallel or anti-parallel to the magnetic moment in the RL depending on the magnetic field direction from the recording media. Digital data sequence made of “0” or “1” is translated into different magnetization directions on the recording media which is recorded by a write sensor in each recording head. As the FL rotates, the resistance measured by passing a current from the FL to the RL will change. The change in resistance is measured and used to decode the magnetization pattern from the recording media and reproduce the information that was recorded earlier.
In FIG. 1A, one example of a TMR reader is shown having a sensor structure 6 formed between a lower shield 4 and an upper shield 7. The down-track cross-sectional view depicts a front side of the sensor structure at an ABS 30-30, and a backside 6e adjoining a dielectric (gap) layer 5b. In a so-called bottom spin valve configuration for the sensor structure, bottom portion 6a comprises a RL, and may also include one or multiple seed layers and an AFM layer on the seed layer (not shown). There is a tunnel barrier 6b between the RL and a FL 6f. Upper portion 6c is a capping layer. In some designs, a part of the RL may be recessed from the ABS.
Referring to FIG. 1B, an ABS view of the TMR reader in FIG. 1A is illustrated and shows magnetization 3m in adjacent JSs 3 provide a longitudinal biasing effect to stabilize FL magnetization 6m in the absence of an external magnetic field. One or both of permanent magnetic material and soft magnetic material each having a magnetization aligned near the FL are generally used to bias the FL magnetization moment with respect to the RL so as to obtain a substantially orthogonal relative orientation between FL magnetization 6m and RL magnetization 6n in a zero applied field environment.
FIG. 1C depicts a top-down view of the sensor structure in FIG. 1B where layers above the FL are removed. A JS 3 is formed adjacent to each side of FL 6f at the ABS 30-30. The longitudinal biasing scheme provides a JS magnetization 3m that is parallel to the ABS and to the FL magnetic moment 6m so that a single domain magnetization state in the FL will be stable against all reasonable perturbations when no external magnetic field is applied. An inner JS side 3s1 is usually separated from the FL by a dielectric layer 5a. Each JS also has a front side at the ABS, a backside 3e, and an outer side 3s2 facing away from the FL. Note that the cross-track direction along the y-axis is known as the longitudinal direction, and a direction orthogonal to the ABS (along the x-axis) is referred to as the transverse direction.
Asymmetry of the quasi-static test (QST) response of a TMR read head is defined as the relative difference in the reader resistance for positive and negative magnetic fields (of equal magnitude) in the transverse direction. QST asymmetry, which is hereinafter referred to as asymmetry, is strongly dependent on the aspect ratio (AR) of the reader, which is defined as the ratio of stripe height (SH) to FL width (FLW) expressed as SH/FLW where SH is the distance between a front side of the FL 6f at ABS 30-30 and the FL backside 6e. Asymmetry increases as AR increases thereby making the reader performance sensitive to process induced AR variations, and establishing an upper limit to allowable AR set by acceptable asymmetry.
Asymmetry of reader response depends, among other factors, on the relative magnetization directions of the FL and the RL in the absence of an applied magnetic field (zero field). Thus, FL magnetization direction in a zero field environment is affected by the strength of the longitudinal bias, given by the magnitude of 3m, and the FL AR. The zero field relative magnetization directions of the FL and RL are quantified using the so-called “bias point”, which is quantified by the resistance of the sensor structure stack at zero field relative to that at a very large external applied field when the FL and RL magnetizations are driven parallel to each other. Because asymmetry becomes considerably large for long SH dimensions that lead to an AR of 1 or higher, and contributes to degraded TMR reader performance, especially when FLW is proximate to 30 nm or less, a method to improve the aforementioned sensitivity to increasing AR or SH is needed.